1. Field of Invention
The present invention relates to an apparatus and corresponding method for stripping off a photoresist layer. More particularly, the present invention relates to an apparatus and corresponding method for stripping off a photoresist layer after a patterned polyimide layer is developed.
2. Description of Related Art
Organic light-emitting display (OLED) is a self-illuminating, high brightness, high contrast, wide viewing angle, low driving voltage and high response rate device. In fact, OLED is a new generation of flat panel display that holds great promise in the near future. However, due to immature techniques , many technical aspects for forming an OLED device remain unresolved. Hence, only one of the Japanese companies, Pioneer, has developed and put up some OLED device in some of their small dimensional products with a small scale mass production.
Photoresist stripping is a process normally used in the last step of a photolithography for producing a semiconductor or display device. The purpose of the photoresist stripping is to remove the photoresist protective cover used in a previous pattern etching so that no residual photoresist is remainined to affect the next process of photolithography. Hence, a clean substrate with circuit pattern thereon is obtained. However, for each photoresist stripping after the formation of, for example, the electrode connection layer, the indium-tin-oxide (ITO) anode layer and the isolation layer on each OLED panel, some residual photoresist material will remain on the OLED panel. This is because the ITO substrate used in the OLED panel has a rougher surface than a conventional epitaxial Si substrate that forms semiconductor circuits. Consequently, residual photoresist can more readily stick on the surface of the ITO panel. Once photoresist remains on the panel surface, especially in the ITO anode region, subsequent vapor deposition of organic light-emitting material and cathode material will be tricky. Dark spots may appear on the display panel and quality of the display may considerably decline. In addition, the subsequently deposited organic light-emitting layer is relatively thin. The non-uniformly distributed residual photoresist on the panel may create a non-uniform electric field that can lead to possible device short-circuiting and decreased the operating lifetime of the device.
In general, a photoresist layer is used to pattern a non-photosensitive polyimide layer. Polyimide is a material having high thermal stability, physical stability, electrical stability and photoelectric stability. Ultimately, the OLED panel with a Polyimide layer can have better light-emitting stability and a longer lifetime. The patterning process includes coating a polyimide or polyimide precursor over a substrate and then coating a photoresist layer over the polyimide layer. A series of operations such as pre-baking, exposure, post-exposure baking, photoresist developing, photoresist stripping, high-temperature baking is sequentially conducted to form a pattern in the polyimide layer. However, the Polyimide is apt to damage by the alkaline stripping agent such as KOH before the ultimate step of high-temperature baking. Hence, the selection of stripping agent and the control of stripping time are significantly important to OLED lithography process. Photoresist stripping in a conventional integrated circuit and photoelectric fabrication process includes spinning a silicon substrate by attaching the substrate onto a suction spinner. While the silicon substrate is spinning, stripping solution and rinsing solution are sprayed simultaneously and continuously. Finally, the silicon substrate is spun dry at a high speed. However, as size of a display panel increases and with glass substrate replacing silicon substrate, a conventional spinning spinner can hardly support such heavy loading and the centrifugal force created at high spinning rate can be dangerous. Therefore, the aforementioned photoresist stripping process is bound to remain some residual photoresist and lead to a great reduction of process yield. An alternative method is to immerse the display panel in various baths containing alkaline solution and then rinsing with water thereafter. Yet, this is a non-continuous process because the display panel has to be taken out, either manually or mechanically, after each immersion treatment. Hence, not only is the total processing time increased, but the operation of large display panel is very troublesome and inconvenient too. Moreover, if the display panels are handled manually in the stripping process, damages caused by delays or human errors are additional factors that must be considered in the continuous mass productionline. In brief, most conventional setup can hardly well control photoresist stripping and ensure an acceptable quality in mass production scale of OLED.
Furthermore, the polyimide used for producing the OLED panel must go through a final baking at a high temperature of between 220xcx9c350xc2x0 C. Only after the final baking step will the polyimide layer have the high electrical, mechanical, thermal and chemical stability an OLED panel required. Before the final thermal baking, chemical stability of polyimide is not well established. Hence, the polyimide material is apt to damages by corrosive alkaline stripping agents. In other words, the alkaline solution such as sodium hydroxide and potassium hydroxide used for stripping photoresist in the conventional technique may often lead to partial stripping and dissociation of the polyimide layer on the OLED panels . To diminish polyimide damage on the OLED panels, timing of the photoresist stripping has to be accurately monitored and controlled, and stripping agents has to be carefully selected. In conclusion, the stripping process of the polyimide layer on the OLED panel has to be designed to increase yield, stability, automaticity and capacity.
Accordingly, one object of the present invention is to provide an apparatus and corresponding method for stripping a photoresist layer off an organic light-emitting display (OLED) panel. The stripping process not only can produce an OLED panel with high degree of surface cleanliness, but also can operate continuously to deal with display panel of various sizes. Hence, the setup and method is suitable for stripping photoresist layer from OLED panel in mass production.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a photoresist stripping apparatus and a corresponding method for removing photoresist material after a polyimide layer on an organic light-emitting display panel is patterned. The photoresist-stripping apparatus includes a transporting unit, a stripping unit, a washing unit, a drying unit and a control unit. The transporting unit connects the stripping unit, the washing unit and the drying unit. The control unit is responsible for controlling the transport sequence and timing of the transporting unit. The apparatus may further include a carrier unit serving as a storage area for holding display panels waiting for photoresist stripping. Alternatively, the carrier unit may serve as a buffer region for storing display panels after photoresist development so that the display panels can be directly transferred to the stripping apparatus. The apparatus may further include a downloading unit serving as a storage area for storing photoresist stripped display panels. Alternatively, the downloading unit may serve as a buffer region for storing the display panels after photoresist stripping so that the display panel can be directly transferred to a subsequent high-temperature baking module instead of a downloading unit.
The transporting unit is responsible for transferring OLED panels into the stripping unit, the washing unit and the drying unit sequentially and continuously. The transporting unit includes a roller unit. Each roller unit further includes an axle and two rollers attached to each end of the axle. Each roller has an inner edge serving as a load support and an outer edge serving as a retainer. The load supporting inner edge is used for supporting OLED panels while the outer edge is responsible for restricting the horizontal movement of the OLED panels supported by the rollers. The OLED panels are transported from one location to another via the rolling action provided by the rollers.
The stripping unit at least includes an n-butyl acetate (NBA) stripping bath and an isopropyl alcohol (IPA) stripping bath. The NBA bath includes a storage tank, a liquid supply system and a reaction chamber. The storage tank is a storage area for n- butyl acetate. NBA liquid in the storage tank is transferred to the reaction chamber through the liquid supply system. The NBA liquid reacts with the OLED panels shuttling inside the reaction chamber to strip off surface photoresist. The shuttling motion of OLED panels inside the reaction serves to even out and quicken the removal of the photoresist material. The liquid supply system injects the n-butyl acetate into the reaction chamber. Through immersion, spraying or partial immersion and spraying of the n-butyl acetate, photoresist on the surface of the OLED panel is gradually dissolved and carried away. The stripping unit includes at least one n-butyl acetate stripping bath. If more than one n-butyl acetate stripping baths are used, these stripping baths may be arranged serially or in parallel or a mixture of both. The advantage of connecting the stripping bath in parallel is that high volumes of OLED panels can be processed at the same time. The advantage of connecting the stripping bath in series is that the same OLED panel can receive consecutive photoresist stripping so that the panel is ultimately much cleaner. In a system having serial and parallel stripping baths, the advantages of both arrangements are obtained. In addition, the deployment of two or more stripping baths can avoid stoppage when n-butyl acetate needs to be flushed from a stripping bath.
The isopropyl alcohol stripping bath is positioned behind the n-butyl acetate stripping bath. The isopropyl alcohol stripping bath at least includes an isopropyl alcohol stripping bath. If more than one isopropyl alcohol stripping baths are used, the stripping baths can be arranged in parallel or in series similar to the n-butyl acetate stripping baths. The isopropyl alcohol stripping bath serves to remove any residual photoresist on the OLED panel surface and displace any residual n-butyl acetate from the surface of the OLED panel. Each isopropyl alcohol stripping bath also includes a storage tank, a liquid supply system and a reaction chamber. The storage tank holds the isopropyl alcohol. Isopropyl alcohol is pumped from the storage tank to the reaction chamber by the liquid supply system so that the OLED panel can be treated. Because isopropyl alcohol has a relatively low boiling point, the OLED panels are treated by immersion rather than by spraying to reduce alcohol vapor generation. In general, the components inside the stripping unit are manufactured using anti-explosion stainless steel for greater safety. An addition waste recycling system may be installed between the n-butyl acetate stripping bath and the isopropyl alcohol stripping bath. In addition, a cooling system may be installed inside the isopropyl alcohol bath to recycle the low boiling point alcohol in line with environmental considerations.
The washing unit is positioned right after the stripping unit. The washing unit is a location where a cleaning solution such as de-ionized water is sprayed on the OLED panels so that any residual isopropyl alcohol is removed. The washing unit has at least one washing bath. Each washing bath includes a storage tank, a liquid supply system and a reaction chamber. The storage tank holds de-ionized water. De-ionized water is pumped from the storage tank to the reaction chamber by the liquid supply system. Any isopropyl alcohol is removed by spraying de-ionized water onto the surface of the OLED panels. In general, a design having two serially connected washing baths are employed so that the OLED panels are washed twice in sequence. In addition, the used de-ionized water in the second washing bath can be reused by collecting and feeding to the first washing bath. In this way, some water resource is saved.
The drying unit is positioned after the washing unit. The drying unit is responsible for removing any residual de-ionized water from the surface of the OLED panels. An air knife blow drying method may be employed in the drying unit.
The control unit is a controlling device responsible for coordinating the sequence and timing of the transporting unit so that appropriate treatments of the OLED panels are provided by the stripping unit, the washing unit and the drying unit. In addition, the control unit may provide a proper engagement of the developing unit and the high-temperature baking unit with the stripping unit, the washing unit and the dry unit. The control unit can also provide some flexibility according to the use of the developing unit and maintenance.
In brief, this invention provides a photoresist stripping device and a corresponding photoresist stripping method. The apparatus and the corresponding method not only can remove a photoresist layer from the surface of an OLED panel with high cleanliness, but can also strip photoresist layer from OLED panels having various sizes en-mass in a continuous process.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.